SLIP INVERSION IN THE FOREARC OF TOHOKU JAPAN, OVER MULTIPLE TIME SCALES

Multiphase deformation and stress histories characteristic of subduction zones lead to the development of a network of brittle faults in the upper plate that can serve as structural weaknesses and accommodate strain in response to changes in boundary conditions at the plate interface. In northeastern Japan, a network of active faults developed as the result of Plio-Quaternary contractional inversion of inherited Miocene extensional faults are active as thrust faults today. Interestingly, following the 2011 Tohoku M9 earthquake, several upper plate faults in the forearc had aftershocks with an extensional slip sense, opposite that predicted from the long term geologic record, suggesting that forearc faults may have geometries and mechanics favorable for stress transfer, earthquake triggering, and slip inversion. However, little is known about the geometry and deformation history of these forearc fault systems. Here we document the Neogene kinematic history and subsurface geometry of the Oritsume, Futaba and Noheji faults in the forearc of Tohoku, Japan. Through field geologic mapping and dating of growth strata, we find evidence for a 5.6-2.2 Ma initiation of Plio-Quaternary contraction along the Oritsume, Noheji and Futaba faults, and document an earlier phase of Miocene extension from 24 to 25 Ma along the Orisume and Futaba faults. Kinematic fault-related fold modeling indicates that these faults have listric geometries, with ramps that dip ~40-65° W and sole into subhorizontal detachments at 6-10 km depth. These data suggest that the forearc contains a network of high angle, listric, basement-involved contractional faults that developed as a result of thrust inversion of Miocene extensional half graben initially formed during the opening of the Sea of Japan. The geometries of these forearc faults make them susceptible to both normal and thrust sense slip if they are mechanically weak relative to the surrounding crust. We suggest that the extension and inversion history in northeast Japan primed the forearc with a network of weak faults that are mechanically and geometrically favorable for reactivation and inversion on geologic timescales, in response to secular variations in stress state associated with the seismic cycle.